Traditional pulmonary function tests such as spirometry, require patient cooperation and thus are generally inappropriate for infants and are often ineffective for young children. The forced oscillation technique, introduced by Dubois et al. (1956), has been used on healthy adults, as well as adults with respiratory disease, to obtain respiratory impedance data over various frequency ranges. Respiratory impedance (Z.sub.in) is defined as the complex ratio ##EQU1## where P.sub.ao and V.sub.ao are the pressure and flow, respectively, at an airway opening. Z.sub.in is often expressed by its real and imaginary components versus frequency (i.e. an impedance spectrum). Presumably, the Z.sub.in spectrum reflects the mechanical properties of the respiratory system. Consequently, mechanical models have been used to interpret adult human Z.sub.in data. For example, healthy Z.sub.in over the frequency range of 2 to 32 Hz allows the estimation of total respiratory system resistance, inertance and compliance by analysis using a three-element model consisting of a resistance, inertance and a compliance in series. For patients with respiratory disease, these properties can change accordingly. Often, the spectral shape itself changes as well, motivating additional model properties and model structure. For example, in adult human subjects, the Z.sub.in data from 5 to 320 Hz includes an airway acoustic anti-resonance which may permit noninvasive insight on airways alone as proposed by Jackson et al (1989). In smaller mammals such as dogs, both tissue specific and airway specific antiresonances were identified in the Z.sub.in data between 2 and 256 Hz (Jackson and Lutchen, 1991). In the past, a six-element model (FIG. 1) has also been used to describe Z.sub.in. The components of the six-element model are divided into airway parameters (R.sub.aw, I.sub.aw) and tissue parameters (R.sub.t, I.sub.t, C.sub.t) which are separated by a shunt compliance (C.sub.g) representing gas compressibility. However, normal infant Z.sub.in data for frequencies of 256 Hz or less may not justify a model of this complexity.
Because the forced oscillation technique is noninvasive and relatively simple, it can be applied to infants. Using the forced oscillation technique, Z.sub.in measurements can be quickly made during quiet breathing with minimal discomfort to the infant. Few studies have reported measurements in non-intubated infants using the forced oscillation technique (See, for example, Desager et al., 1991; Marchal et al., 1989; Hordvik et al., 1985; Nussbaum and Galant, 1984; Wohl et al., 1969). However, these measurements were typically constrained to low frequencies (e.g., f&lt;40 Hz). Also, unlike adult Z.sub.in data, these data required that a mask be placed over the infants oral and nasal cavities and that the oscillations be presented through the nose. As a consequence, it remains heretofore unresolved as to how the mask and specific physiologic properties contribute to the Z.sub.in spectrum.
While various models have been used in the prior art, none have specifically taken into account the effect of the mask on the Z.sub.in data. This is obviously a drawback which may lead to erroneous results.